3. FEATURES OF AGN SPECTRAL ENERGY DISTRIBUTIONS (SEDs)

The notable, standard features in quasar SEDs are labelled in
Figure 1. Both radio-loud (RLQs) and
radio-quiet quasars (RQQs)
typically show bumps in the optical-UV and in the IR with an inflection
point at ~ 1µm between the two. This inflection point, which
represents a minimum in the energy output of the quasar, lines
up with the peak in the starlight contribution from the host galaxy. In
the lower luminosity, Seyfert galaxies the host galaxy contribution becomes
comparable to the emission from the active nucleus, flattening the
optical-IR
SED and resulting in an apparent decrease in the strength of the optical and
IR bumps. The dominance of the host galaxy emission
~ 1µm in these objects has been exploited to
study their host galaxies
(McLeod & Rieke 1994a,
b,
Taylor et al.
1996,
Dunlop et al.
1993).

Figure 2 shows the median SED for RLQs (dashed)
and RQQs
(solid) from EWM94 showing the remarkable similarity in the IR-UV region.
A similar figure was presented by
Sanders et al.
(1989)
for the PG quasar sample. Although these medians look very similar and
well-behaved,
there is a large dispersion around the median which fills
most/all of the available parameter space
(Figure 2, lower plot).
This dispersion exacerbates searches for systematic
dependencies of the SEDs on other quasar properties (luminosity,
Wilkes et al.
1999)
and needs to be included
in any interpretation and modelling of the SEDs.

The near-IR bump generally peaks ~ 25 - 60µm. It is broad,
decreasing slowly to > 100µm, the longest wavelength
accessible to IRAS.
New observations by ISO should extend this coverage to 200
µm, but
problems with the calibration have so far prevented reliable
results. A recent paper from the ISOPHOT team which reports
far-IR SEDs for 10 RLQs and RQQs suggests
that this situation will soon improve
(Haas et al. 1998).

Figure 3. The radio-UV SED of the
radio-quiet quasar, PG1351+640.
The combination of the FIR flux from IRAS and the mm upper limit
yeild an estimate for
fir > 2.4
(f-fir), close to
the limit for a pure, homogeneous synchrotron source and so favoring
thermal, dust emission in the far-IR.

RLQs fall into two distinct groups generally believed to
be distinguished by the orientation of the source to our line-of-sight.
Core-dominated (CD) RLQs are those in which relativistically
beamed, synchrotron emission is pointed directly at us and so
is boosted. In these sources, the radio-IR SED tends to be smooth,
suggesting that the IR is a higher-energy extension of the synchrotron
emission in the radio. In Figure 4 the mean
SED of CDRLQ 3C273 from the compilation of 30 years of
multi-wavelength monitoring by
Tuerler et al.
(1998)
is displayed. The SED extrapolates smoothly from the radio
into the IR and is observed to vary in a coordinated manner
(Courvoisier 1998),
strongly arguing for non-thermal emission throughout the radio-mid-IR SED.
In the near-IR,
~ 3µm, there is a narrow peak of emission which does not
vary in the same way and is believed to originate in hot dust
(Courvoisier 1998).
Even in this well-established, non-thermal source,
dust emission is an important contributor.

Figure 4. The average
radio--ray SED of the
core-dominated, radio-loud quasar 3C273 shown as a:
log(F) and b:
log(
F)
vs log() (from
Tuerler et al.
1998,
their Figure 6).
The smooth extrapolation of the SED from radio into IR combined with
correlated variability throughout the radio-mid-IR, strongly
argue for a common/related, non-thermal emission mechanism in this
spectral region.

Lobe-dominated (LD) RLQs are dominated by extended emission
on either side of the central AGN, powered
by the relativistic jets which, in these sources, are not close to our
line-of-sight. Studies of the radio-FIR SEDs of the core emission
have revealed a discontinuity in the mm region which implies
that emission in the FIR is unrelated to the non-thermal radio emission
(Antonucci, Barvainis
& Alloin 1990)
and so favors a thermal origin for the FIR.

The radio properties of RQQs are typically either unresolved or with
double, triple or linear structure
providing strong evidence for jet-producing
central engines similar to those in RLQs
(Section 4.1.).

There is a second "bump" in typical quasar SEDs, the big blue bump (BBB),
which dominates the
optical-UV emission and often their total energy output. Since the X-ray
emission is generally below an extrapolation of this BBB, it is
generally thought to peak in the EUV region. Recent, high-quality,
UV data (Figure 5,
Zheng et al. 1997)
shows evidence
for a peak in the far-UV in low-redshift objects. The detection of a
peak is a major
step forward as it allows constraints to be placed on the EUV emission,
which is the source of most of the ionising photons,
and on the relation between the UV and soft X-ray emission.
At high redshift a similar measurement is difficult due to the
affects of intervening absorption on the continuum shortward of the
Ly emission line.

Although the slopes generally steepen towards lower energies (0.1-2 keV,
ROSAT band), RLQs remain relatively flatter than RQQs
(Laor et al. 1997).
These authors also suggest that this spectral difference may be linked
to the width of the broad
H4861 emission line rather than
the radio emission.

A fluorescent, Fe K emission
line is commonly observed, again tending to be stronger in low
luminosity sources
(Nandra et
al.1997a,
b).
This Fe K line is often
broad, and sometimes double-peaked.
consistent with its origin in an accretion disk and, if so, providing
the only direct observations of this accretion disk to date
(Tanaka et al.
1995,
George & Fabian 1991,
Nandra these proceedings).

At soft energies,
1 keV, the X-ray
spectra of ~ 50% of
both radio-loud and radio-quiet AGN steepens significantly to form the
soft excess
(Masnou et al.
1992).
This component generally dominates their
ROSAT (0.1-2 keV) spectra leading to systematically steeper
slopes at these energies
(Fiore et al. 1994,
Buehler et al.
1995).
The soft
excess can be extremely steep and is often called the ultra-soft excess
(Puchnarewicz et
al. 1992,
Boller, Brandt & Fink
1996).
Given the general increase of both the BBB on the low
energy and the ultra-soft excess on the high energy side of the EUV region,
it is tempting to identify both as part of the same, BBB component.
There is little direct evidence to this effect, for example observations of
coordinated variability, but circumstantial evidence includes the
anti-correlation of optical and soft X-ray slopes
(Puchnarewicz et
al. 1996)
and the extrapolation of the soft X-ray into the UV region
(Laor et al. 1997).

Studies of X-ray slopes as a function of redshift show changes which
correspond well to these various components moving through the observed
band as the redshift increases. Comparison of the slopes
using a constant intrinsic energy range shows no dependence of
x on redshift
(Elvis et al. 1994,
Cappi et al. 1997).

Another key component of X-ray spectra of quasars and Sy galaxies is
absorption by cold and/or warm material.
Sy1 galaxies and quasars typically show low absorption column densities
(NH
~ 1020 - 21 cm-2) often including ionised
absorption edges indicating the presence of warm material
(Halpern 1984).
This warm material has been suggested to originate in
the same material as that responsible
for the narrow associated UV absorption lines common in many types of AGN
(Mathur, Elvis &
Wilkes 1995
and references therein).